In eukaryotes facultative heterochromatinization (FH) may serve for the genetic inactivation of whole chromosomes. The best known case of FH is that involving one of the two X chromosomes in the mammalian female. The proposed study will analyze the two other known cases: the FH of the X during spermatogenesis, and that of a whole set of chromosomes in male mealybugs. The FH of the X during spermatogenesis is very widespread and it has been suggested that it gave rise to the other two cases and thus that in all three the underlying molecular mechanism is similar. The object of the study is to learn about this mechanism by determining the features common to all three cases. The FH of the X during spermatogenesis will be studied in the grasshopper Melanoplus femur-rubrum in which I recently determined that in tetraploid spermatocytes only one of the Xs present is heterochromatic. The behavior of these Xs is thus similar to those of the mammalian female. I also determined that these tetraploid spermatocytes are arrested during telephase I. The possibility that the arrest of these cells was due to the genetic activity of the euchromatic X will be examined using tritiated uridine and autoradiography. The FH of a whole set of chromosomes will be analyzed in the mealybug Pseudococcus obscurus. The heterochromatic (H) set is of paternal origin and is normally not transmitted. It is planned to transmit some of the H chromosomes into female embryos by translocating them onto a B chromosome, and then to determine whether the chromosomes remain heterochromatic or whether they become euchromatic. The role of the euchromatic chromosomes in maintaining the paternal set in the heterochromatic state will be analyzed by determining the behavior of the paternal set in the haploid sector of male embryos which are haplo-diploid mosaic.